1. Field of the Invention
The present invention relates generally to oil well drilling. More particularly, the present invention relates to determining the quality of the bonding between a downhole casing and the surrounding formation. Most particularly, the present invention relates to a new system and method for obtaining an impedance measurement of the cement bonding between a downhole casing and the surrounding formation.
2. Description of the Related Art
Although oil well production has been a boon to mankind, with it has come certain deleterious side effects. One problem known in oil field drilling is the contamination of fresh water aquifers by hydrocarbon deposits explored by a wellbore. That is, when an oil well borehole is drilled through both an aquifer and a hydrocarbon region, there exists a risk that hydrocarbons will flow from the hydrocarbon region to the aquifer. This contamination can have a significant negative impact on the environment and upon drinking water.
FIG. 1 illustrates the occurrence of such contamination. The surface of the earth 100, a borehole 110 passing through a rock lithology 160 including an aquifer region 120 and an oil region 130, a steel casing 140 residing in the borehole, and cement 150 bonding the casing 140 to the surrounding formation 160 are pictured. The steel casing 140 is provided around the periphery of the borehole 110 after drilling the well, in part to prevent the collapse of the well. This steel casing is held in place by special cement 150. If the cement has proper bonding, the risk of oil flow 170 from the hydrocarbon deposits to the aquifer region is reduced significantly. If, however, the casing is improperly bonded, the risk that contamination may occur is much greater.
It has been found that a casing's bonding properties correspond to the impedance of the cement bonding the casing to the surrounding formation. The cement impedance indicating proper bonding varies depending upon the type of cement used. Nonetheless, where the cement has an impedance of less than 3, there exists poor bonding. At about an impedance of 3, adequate bonding has been achieved. Preferable bonding, however, has an impedance on the order of 4 to 5 for soft cement, and 5 to 6 for hard cement. Therefore, an invention capable of determining the impedance of the bonding would be useful to those searching for ways to determine the bonding properties. Further, such an invention could be used to evaluate cementing techniques so that more effective methods of cementing the casing to the formation may be developed.
Some efforts have been made to determine by acoustic tools the bonding properties of casing. Nonetheless, there still exists a need for an alternative, fast, and reliable device and method to determine the quality of the bond between the casing and the formation surrounding the borehole.